Process for preparing platinum-tin reforming catalyst and use thereof

ABSTRACT

Supported platinum group metal-tin hydrocarbon conversion catalysts are prepared by (1) impregnating a refractory inorganic oxide support with an aqueous solution of a halogen acid containing a platinum group metal component; (2) drying the platinum group metal impregnated carrier; (3) impregnating the dried platinum group metal impregnate with a solution of a divalent tin compound in a non-oxidizing and non-reducing atmosphere; and (4) drying the resulting impregnate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to (a) a process of preparing ahydrocarbon conversion catalyst, particularly a catalyst useful inpetroleum refining and petrochemical processing, such as hydrocarbonreforming operations; (b) an improved hydrocarbon conversion catalyst;and (c) an improved naphtha reforming process. More particularly, thisinvention relates to an improved catalyst comprising a complex compoundof a platinum group component and a tin component associated with arefractory inorganic oxide carrier, a process for preparing the improvedcatalyst, and to the use of the improved catalyst in a naphtha reformingprocess.

2. Description of the Prior Art

Platinum-tin hydrocarbon conversion catalysts have been describedpreviously in the literature. Netherlands application No. 70,13354, andU.S. Pat. Nos. 3,632,525; 3,584,060; 3,679,601; 3,725,304; 3,631,215;3,511,888 and 3,531,543 contain disclosures relating to hydrocarbonconversion catalysts containing platinum group metal and tinconstituents. Additionally, we, in Japanese Patent Application No.243337/1971, have disclosed the existence of catalysts comprising asupported platinum-tin composite. Finally, the existence of complexcompounds of platinum chloride and tin chloride and their use aspromoters for the hydrogenation of olefins of fatty acids in ahomogenous system has been reported in the Journal of the AmericanChemical Society, Volume 25, 1681 (1963).

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been discovered thathighly effective hydrocarbon conversion catalysts, in particular naphthareforming catalysts, comprising a supported complex compound of aplatinum group metal constituent and a tin constituent can be securedutilizing the preparation technique of the present invention. Thepreparation technique comprises impregnating a refractory inorganicoxide carrier or support with an aqueous solution of a halogen acidcontaining a platinum group metal component, drying the resultingplatinum group metal-refractory inorganic oxide impregnate, impregnatingthe platinum group metal component-refractory inorganic oxide impregnatewith a solution of a divalent tin compound in a non-oxidizing andnon-reducing atmosphere; and drying the resulting impregnate. Followingthe second drying operation it is preferred that the completedimpregnate be calcined at elevated temperature to form thereby, in situ,a complex compound of the platinum group metal component and the tincomponent on the surface of the carrier material. While the catalyst maybe employed to promote a wide variety of hydrocarbon conversionprocesses, the catalyst has particular utility in promoting theconversion of naphtha materials to higher octane gasoline products.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1 and 2 illustrate the relationship between the temperaturenecessary to maintain a constant product octane level in a naphthareforming operation and duration of on-oil contacting.

FIG. 3 illustrates the relationship between gasoline yield in a naphthareforming process and duration of on-oil contacting.

FIG. 4 demonstrates the decline of rate constants of benzene formationwith time in a normal hexane dehydrocyclization reaction.

Any of the usual refractory inorganic oxides may be used as the carrieror support constituent of the catalyst of this invention. For instance,there may be used such refractory carriers as alumina, bentonite, claydiatomaceous earth, zeolite, silica, magnesia, zirconia and thoria. Theuse of alumina is especially preferred. Mixtures of alumina with minoramounts, e.g., from about 1 to 20 percent by weight, of other refractorymaterials may be used. Specifically, alumina-silica, alumina-zirconia,alumina-silica-magnesia and the like may be employed. It is preferredthat the carrier be a porous substance having a specific surface area ofat least about 50 square meters per gram. Preferred examples of aluminathat are useful as the catalyst support are gamma-alumina, eta-alumina,and chi-alumina. These materials have a specific surface area of atleast about 130 square meters per gram, a bulk density of about 0.3 toabout 0.8 grams per milliliter, an average pore volume of about 0.3 to1.2 milliliters per gram and an average pore diameter of about 40 to 300A.

Useful platinum group metals belong to Group VIII of the Periodic Tableand include light platinum group metals such as ruthenium, rhodium andpalladium and heavy platinum group metals such as osmium, iridium andplatinum. The use of platinum is especially preferred. In theimpregnation operation, it is preferred that the platinum group metalsbe used in the form of soluble compounds. The impregnation solutioncontaining the platinum group metal component is prepared by dissolvinga soluble compound of the platinum group metal in an aqueous solution ofa halogen acid (a hydrogen halide). The chemical indentity of thesoluble compound used is not particularly critical and any of thesoluble compounds may be employed. However, use of halogen-containingsoluble compounds is particularly preferred. Useful platinum componentsinclude platinum nitrate, halogenplatinic acid, and ammoniumhalogenplatinate, platinum halide, etc. Chloroplatinic acids, such ashexachloroplatinic acid and tetrachloroplatinic acid, ammoniumchloroplatinate and the like are particularly preferred. It is alsopossible to employ a polyamine salt of platinum.

The impregnating medium is preferably a dilute aqueous solution ofhydrochloric acid and it is desired that the concentration of acid inthe solution be maintained between 0.005 to 0.5 normality, especiallybetween 0.01 and 0.3 normality with a pH of less that 4, preferably lessthan 3. The acid concentration in the aqueous impregnating solution hasan inlfuence on the degree to which the active ingredient enters intothe interior of the support or carrier from the surface thereof.

The concentration of the platinum group metal component in theimpregnation solution is dependent upon the amount of platinum groupmetal desired to be supported on the carrier as well as other factors.It is generally preferred that the concentration be maintained withinthe range of from about 0.005 to 0.05 moles per liter, calculated aselemental metal.

The impregnation solution containing the tin component may be an organicor aqueous solution of a divalent tin compound, preferably a stannoushalide, most preferably stannous chloride. It is possible to employ animpregnating solution comprising a mixture of an organic solution of thetin compound and an aqueous solution of the tin compound. Useful organicsolvents include alcohols, ethers, and ketones. The use of an alcohol,especially a lower aliphatic alcohol, such as methanol, is preferred.When an aqueous solution of the tin component is employed, it isdesirable that the aqueous solution contain a halogen acid (a hydrogenhalide) in a concentration varying from about 0.002 to 0.3 normality. Adilute aqueous solution of hydrochloric acid is a particularly usefulmedium for the tin component. When an aqueous solution of a halogen acidis used, the pH of the solution is adjusted to less than about 6,preferably less than about 5. Typically, the tin concentration of theimpregnation solution is maintained within the range of from about 0.001to 0.1 moles per liter.

The impregnation operation of the present invention is conducted byadmixing the carrier with the aqueous solution of the above-mentionedplatinum group metal component to produce an impregnate containing from0.01 to about 3.0 percent by weight of platinum group metal based uponthe weight of the catalyst. Typically, the carrier is in contact withthe impregnation solution at room temperature for more than about 10hours; however, other conditions may be employed to produce animpregnate that contains the above-mentioned quantities of platinumcomponent. The platinum group metal containing impregnate is thenseparated from the impregnation solution by filtration or by using othersimilar processes. The impregnate is then washed to remove anyunnecessary accumulations therefrom and dried using process conditionsdescribed hereinafter.

The dried platinum group metal impregnate is subjected thereafter toimpregnation with the tin component. It is essential that the tinmaterial of the impregnating solution be maintained at a valence of 2and the impregnation is conducted accordingly in an inert atmosphere toprevent odixation and/or reduction of the tin constituent. Divalent tincomponents in the impregnating solution, for example stannous chloride,is oxidized readily to change its valence state from 2 to 4 in anoxidizing atmosphere and is reduced to a valence of 0 in a reducingatmosphere. Oxidation or reduction of the tin constituent prevents theformation of the desired highly dispersed complex of the platinum groupmetal component and tin halide on the support wth the result that thedesired high catalytic activity of the resulting product will not besecured. Accordingly, the impregnation of the platinum group metalimpregnate with the tin component is conducted preferably in an inertgas atmosphere, such as nitrogen. The tin halide impregnation operationis conducted in the manner as the platinum group component impregnationto introduce from 0.05 to 10 percent by weight, based on total catalyst,of the tin component into the impregnate. Following the tinimpregnation, the impregnate is separated from the impregnation solutionusing conventional techniques and thereafter washed. An alcohol ispreferably used as the washing medium. The washed catalyst is thereafterdried and calcined. The final catalyst, prepared in the mannerindicated, will contain halogen, preferably chlorine, in the range offrom about 0.1 to 5 percent by weight, based on total catalyst. Ifdesired, additional amounts of halogen, preferably chlorine, can beintroduced into the catalyst using conventional techniques.

The drying step conducted after each of the impregnation steps of thecatalyst manufacturing process is carried out by heating the impregnateat a temperature in excess of about 80° C., preferably at a temperaturevarying from 80° to 100° C. in a nitrogen or air atmosphere. If desired,the catalyst can be dried using a filtration-type process at temperaturebelow about 80° C. The final calcination operation is conducted byheating the final impregnate at a temperature ranging from about 400° toabout 700° C.

The hydrocarbon conversion catalyst of the present invention may beemployed in various physical forms. For example, the catalyst may beemployed in the shape of spheres, tablets, cylindrical extrudates or thelike. It is usual that the carrier or support material be formed intothe desired shape prior to impregnation and subjected to a drying andcalcination operation, preferably at the temperature conditionsdescribed above, prior to its being subjected to the impregnation step.

The active component of the platinum group metal-tin halide complexcatalyst of this invention, is in the case of platinum, believed to havethe following structure: ##STR1## in which X designates a halogen atomand Y represents a halogen atom or other constituent.

As can be seen from the above structure, the mole ratio of tin toplatinum group metal in the catalyst complex is believed to be 2.0 andthe tin is believed to be in a divalent form. Although it is not clearwhy the composition prepared as indicated possesses excellent catalyticcharacteristics, they may be attributed to the fact that the complex ofthe platinum group metal and tin halide is highly dispersedsubstantially in the form of a unit compound on the support materialwhich has a large specific surface area and that the ligand of thecomplex gives a weak acidic point and, at the same time, prohibits thecombination of the platinum group metal atoms present with each otherthus preventing the reduction of active points due to agglomeration andcrystalization.

As noted earlier, the catalyst of the present invention is especiallysuited as a promoter for naphtha reforming processes. In the naphthareforming process, the naphtha feed stock and hydrogen are contactedwith the catalyst at a temperature varying from 300° to 600° C.,preferably from between 350° and 550° C., at a pressure less than 15kilograms per square centimeter, preferably at a pressure less than 10kilograms per square centimeter, at a space velocity ranging from 0.1 to10 weight parts of feed stock per hour per weight of catalyst (W/H/W),preferably at a space velocity varying from about 1 to 5 W/H/W and at ahydrogen to hydrocarbon mole ratio ranging from about 0.5 to 20,preferably between 1 and 10. It is preferred that the hydrogen employedin the reforming process be of at least 80 percent purity. Preferably,the process feed stock is a light hydrocarbon oil such as naphthafraction or a kerosene fraction, that is materials that boil atatmospheric pressures at temperatures in the range of from 30 to 350° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

Chloroplatinic acid (6.35 grams of H₂ PtCl₆.6H₂ O) was dissolved in 2liters of a 0.1 normal aqueous hydrochloric acid solution. Four hundredgrams of commercially available alumina carrier having a specificsurface area of 180 square meters per gram and in the form of extrudatesof 1.5 m/m in diameter was calcined in a muffle furnace at 500° C. for 2hours and was impregnated thereafter with the chloroplatinic acidsolution for 48 hours. The impregnate was then filtered, washed, driedat 100° C. for 3 hours and then calcined in a muffle furnace at 500° C.for 2.5 hours. The catalyst, which contained about 0.6 wt.% platinum,was designated Catalyst A.

Stannous chloride (6.23 grams of SnCl₂.2H₂ O) was dissolved in twoliters of methanol. A batch of platinum impregnate identical to CatalystA, except that it was not subjected to the final calcination step, wasimpregnated with the entire stannous chloride solution in a nitrogenatmosphere. The formation of a complex between the platinum and tinchloride was noted by the change of color of the alumina surface fromlemon yellow to a yellowish orange. The stannous chloride impregnationwas carried out for 48 hours. Following the impregnation, the methanolsolution was removed from the impregnate, the catalyst washed withmethanol, dried at 100° C. for 3 hours and then calcined in mufflefurnace at 500° C. for 2 hours. This catalyst was designated Catalyst B.Catalyst B consisted of about 0.6 wt.% platinum and about 0.73 wt.% tinto provide a platinum:tin mole ratio of about 1:2.0.

Chloroplatinic acid (5.40 grams of H₂ PtCl₆.6H₂ O) was dissolved in twoliters of methanol and 5.7 grams of stannous chloride (SnCl₂.2H₂ O) wasadded to that solution to form a yellowish orange platinum-tin chloridecomplex. Four hundred grams of the same calcined alumina carrier thatwas employed in the preparation of Catalysts A and B was impregnatedwith the above complex solution. After a 48 hour impregnation period,the methanol solution was removed from the impregnate, the catalystdried at 100° C. for 3 hours and then calcined in a muffle furnace at500° C. for 2 hours. This catalyst was designated Catalyst C. Catalyst Ccontained about 0.51 wt.% platinum and 0.75 wt.% tin to provide aplatinum:tin mole ratio of about 1:2.4.

Samples of Catalysts A, B, and C were then tested in naphthahydroforming service wherein each catalyst was contacted with hydrogenand a straight run naphtha fraction from a Middle East crude and havinga boiling range at atmospheric pressure of about 90° to 175° C. and asulfur content of 0 percent. The processing was carried out at areaction zone pressure of 10 kilograms per square centimeter, a hydrogento hydrocarbon mole ratio of 8.0 and a space velocity of 1.5 W/H/W. Thereaction zone temperature (means temperature of the catalyst bed) wasmaintained at a level necessary to maintain the research octane numberof the reformate product at 101.5. The tests were carried out for aperiod of from 9 to 10 days and the results thereof are set forth inFIG. 1. From FIG. 1, it is apparent that Catalyst B has a higheractivity than either Catalyst A or C in that the temperature needed toproduce the desired octane level is considerably below that required forCatalyst A and C. Further, Catalyst B has excellent activity maintenancecharacteristics since the rate of temperature increase needed tomaintain the high octane product level is less than Catalyst A.

EXAMPLE 2

Chloroplatinic acid (3.19 grams of H₂ PtCl₆.6H₂ O) was dissolved in twoliters of a 0.1 normal aqueous hydrochloric acid solution. Four hundredgrams of the same calcined alumina support employed in Example 1 wasimpregnated with the above solution for 48 hours. The impregnatedcatalyst was then recovered by filtration, washed, and dried at 100° C.for 3 hours and calcined in a muffle furnace at 500° C. for 2 hours. Thecatalyst, which contained about 0.32 wt.% platinum, was designatedCatalyst D.

A catalyst, identical in amount and preparation to Catalyst A exceptthat the final calcining operation was not performed, was impregnatedfor 48 hours in a nitrogen atmosphere with a stannous chloride solutioncomprising 3.50 grams of SnCl₂.2H₂ O dissolved in two liters ofmethanol. The resulting impregnate was removed from the methanolsolution, washed with methanol, dried at 100° C. for 3 hours and thencalcined in a muffle furnace at 500° C. for 2 hours. This catalyst,which contained 0.32 wt.% platinum and 0.46 wt.% tin and exhibited aplatinum to tin molar ratio of about 1:2.36, was designated Catalyst E.Following the procedure or Example 1, Catalysts D and E were tested forcatalyst activity and catalyst activity maintenance characteristics in anaphtha reforming environment. As in Example 1, the reforming tests werecarried out at a temperature (mean temperature of the catalyst bed)sufficient to maintain a product octane number of 101.5. Additionally,yield measurements were made during the course of the test period todetermine C₅ + yield decline over the course of the run. The results ofthe tests are set forth in FIGS. 2 and 3.

FIG. 2 illustrates that the catalyst of the present invention (CatalystE) is superior to a conventional platinum catalyst in both activity andactivity maintenance characteristics. FIG. 3 illustrates that the yielddecline experienced with the catalyst of the present invention is lessthan that experienced with a conventional platinum catalyst and that theselectivity to the desired C₅ + product is much greater for the catalystof the present invention than for a conventional platinum catalyst.

EXAMPLE 3

To demonstrate the criticality of employing an inert atmosphere duringthe impregnation of the tin halide, Catalyst E and a catalyst similar toCatalyst E but which was prepared contrary to the teachings of thepresent invention (Catalyst F) were compared for their ability topromote the dehydrocyclization of normal hexane to benzene. Catalyst Fwas prepared in exactly the same manner as Catalyst E with the exceptionthat the contacting of the platinum impregnate with the stannouschloride solution was conducted in air rather than nitrogen. Thedehydrocylization experiment was conducted at atmospheric pressure bycontacting a sample of the catalyst with normal hexane and hydrogen at500° C. at a hydrogen to hexane mole ratio of 10 and at a space velocityof 1.0 W/H/W. During the onstream period the rate constant of benzeneformation (mole/hour/mole) was calculated periodically for each of thetwo catalyst systems. The rate constant, in each instance, wasdetermined by dividing the benzene concentration in the product(mole/mole) by the residence time of the normal hexane (hours). Theresults of the tests are depicted in FIG. 4. FIG. 4 illustrates that theactivity and stability characteristics of the catalyst preparedfollowing the procedure of the instant invention are markedly superiorto the catalyst containing the identical constituents that were preparedby contacting a platinum/alumina impregnate with stannous chloride inair rather than in a non-oxidizing and non-reducing atmosphere.

What is claimed is:
 1. A process for preparing a hydrocarbon conversioncatalyst comprising a complex compound of a platinum group metalcomponent and a tin component contained on a refractory inorganic oxidecarrier which comprises:a. impregnating said refractory inorganic oxidecarrier with an aqueous solution of a halogen acid containing a platinumgroup metal component; b. drying the platinum group metalcomponentimpregnated carrier; c. impregnating the dried platinum groupmetal component-impregnated carrier with a solution of a divalent tincompound, said impregnation conducted in a non-oxidizing andnon-reducing atmosphere; d. drying the carrier impregnated with saidplatinum group component and tin component; and e. calcining the driedimpregnate of step (d) at elevated temperatures.
 2. The process of claim1 wherein said divalent tin component is stannous chloride.
 3. Theprocess of claim 1 wherein said aqueous solution employed in step (a)contains hydrochloric acid.
 4. The process of claim 1 wherein saidplatinum group metal component employed in step (a) is chloroplatinicacid.
 5. The process of claim 1 wherein said solution employed in step(c) in an aliphatic alcohol solution.
 6. The process of claim 1 whereinsaid carrier is alumina.
 7. The process for preparing a hydrocarbonconversion catalyst comprising a complex compound of a platinumcomponent and stannous chloride contained on alumina which comprises:a.impregnating the said alumina with an aqueous solution of hydrochloricacid containing a platinum component; b. drying said platinumcomponent-alumina impregnate; c. impregnating the dried platinumcomponentalumina impregnate with a solution of stannous chloride in anon-oxidizing and non-reducing atmosphere; d. drying the impregnate fromstep (c); and e. calcining said dried impregnate from step (d) byheating the same at a temperature varying from 400° to about 700° C. 8.An improved hydrocarbon conversion catalyst comprising a complexcompound of a platinum group component and a tin component contained ona refractory inorganic oxide carrier, said catalyst prepared by aprocess comprising:a. impregnating said refractory inorganic oxidecarrier with an aqueous solution of a halogen acid containing a platinumgroup metal component; b. drying the platinum group metalcomponentimpregnated carrier; c. impregnating the dried platinum groupmetal component-impregnated carrier with a solution of a divalent tincompound, said impregnation conducted in a non-oxidizing andnon-reducing atmosphere; and d. drying the carrier impregnated with saidplatinum group component and tin component; and (e) calcining the driedimpregnate of step (d) at elevated temperatures.
 9. A naphtha reformingprocess which comprises contacting a naphtha feed stock and hydrogenunder reforming conditions with a catalyst comprising a complex compoundof a platinum group metal component and a tin component contained on arefractory inorganic oxide carrier, said catalyst prepared by the methodcomprising:a. impregnating said refractory inorganic oxide carrier withan aqueous solution of halogen acid containing a platinum group metalcomponent; b. drying the platinum group metal componentimpregnatedcarrier; c. impregnating the dried platinum group metalcomponent-impregnated carrier with a solution of a divalent tincompound, said impregnation conducted in a non-oxidizing andnon-reducing atmosphere; and d. drying the carrier impregnated with saidplatinum group component and tin component; and e. calcining the driedimpregnate of step (d) at elevated temperatures.